A superconductive connecting device and also a method for the production of the connecting device are disclosed in WO 2007/128635 A1.
Superconductors using LTC (Low-Tc) superconductor material or HTC (High-Tc) superconductor material are produced amongst other things as so-called single-core conductors or multifilament conductors with a limited conductor length. With regard to a corresponding conductor construction of such conductors, the at least one superconductive conductor lead (or the at least one superconductive conductor filament) is embedded into a matrix made of normally conductive material. In particular during the construction of superconductive devices employing such superconductors such as for example magnet windings, it is necessary to contact or connect end pieces of corresponding conductors.
Some superconducting magnet windings, for example for magnetic resonance tomographs (also referred to as nuclear spin or NMR tomographs), need to be operated in so-called “persistent current mode”. To this end, the magnet winding is short-circuited and the magnet current once set continues flowing without a current source for a practically unlimited period of time. This does however require that the energized circuit of the magnet winding has practically no electrical resistance. To this end, superconductive connections between individual superconductive conductors (superconductors) are also necessary. These superconductive connections do however react sensitively to magnetic fields. That is to say, if the magnetic field or the magnetic flux density exceeds a certain limit value, which is typically between 1 Tesla and 2 Tesla, then the connection exhibits an electrical resistance and thereby renders persistent current operation impossible. With regard to superconducting high field magnets, the magnetic flux density of which is for example 3 Tesla or higher, this results in problems because the connections must be placed at positions in which the local magnetic field lies below the stated limit value. In the case of very high fields, it can even become impossible.
A known superconductive connection for the end pieces of two superconductors is follows for example from “IEEE Transactions on Applied Superconductivity”, Vol. 9, No. 2, June 1999, pp. 185 to 187. In order to establish this connection, at the end pieces to be connected of the superconductors the filaments thereof are exposed for example by means of etching and are then connected to another with the aid of a superconductive solder as a contacting material. As a general rule, lead compounds are employed as solders, for example made from the alloy Pb27—Bi50—Sn12—Cd10 (so-called “Wood's metal”) or similar solders based on a Pb—Bi or Pb—Bi—Sn alloy. All these solder materials have an upper critical magnetic field Bc2 at most about 2 Tesla at a temperature of 4.2 K, the temperature of liquid helium at normal pressure. With regard to fields above this critical magnetic field, they lose their superconductive properties and are therefore frequently not suitable for the design of superconductive high field contacts.
A further connection technology for the end pieces of two superconductors can be found in “Cryogenics”, Vol. 30 (Supplement), 1990, pp. 626 to 629. In this case, the superconductors are connected or pressed together with one another by means of spot welding or also diffusion bonding directly through the application of pressure and temperature without an intermediate material providing/promoting contact. The current carrying capacity of corresponding connections is however as a general rule less than that of the conductor itself. In this situation, the problem occurs that the filaments do not form large surface area contacts but rather point contacts. As magnetic field strengths increase, the current carrying capacity is then however further reduced, which means that connections of this type are also not suitable for many superconductive high field contacts.
A method can be found in DE 34 13 167 A1 for the production of a superconductive contact between superconductors, wherein at conductor end pieces made from a conductor precursor the conductor leads stripped of the matrix material are subjected to a pressure and temperature treatment process in a sleeve together with a certain powder material as an intermediate material. In this situation, the powder material is chosen such that superconductive material is formed with it during this treatment process and also from the conductor precursor. High temperatures in excess of 600° C. for example are however required in this situation. The known method therefore has a very high resource requirement and frequently cannot be employed.
A superconductive connection of end pieces of two superconductors and a method for the production thereof are known from WO 2007/128635 A1, wherein the connection can be produced below 600° C. through the use of MgB2. In this situation, etching steps are used wherein a matrix material in which the MgB2 conductor leads are embedded is removed. With regard to the etching step in which the MgB2 material is exposed and is able to react with the ambient atmosphere, the MgB2 material degrades and becomes brittle. It can crumble following the etching step, which renders a connection of the end pieces difficult or impossible.